1. Field of the Invention
The present invention relates to data transmission and, in particular, to the transmission of data by, for example, voiceband modems, at selectable bit rates. More specifically, this invention pertains to a method and apparatus useful for data transmission using quadrature amplitude modulation and, more particularly, to a method and apparatus which may be optimized to a particular communication channel by using fractional rate modulation.
2. Description of the Prior Art
It is well known in the art of signal communications that the optimum performance of a communication channel is achieved when the data rate of the channel is equal to B.times.log.sub.2 S/N, where S/N is the signal-to-noise ratio and B is the bandwidth. However, data communication systems are also constrained by a number of other parameters which make it very difficult to operate at the idealized bandwidth described above. For example, data communication systems using quadrature amplitude modulation (QAM) are typically constrained by standardized data rates, power levels and bit rates to a ratio of the bits/baud having to be an integer. As a result, in general the data rate of a QAM system is usually lower than the channel capacity.
Methods of fractional rate encoding are known. These include multidimensional trellis codes and a simple encoded method identified in a reference below. See, for example, Forney et al, "Efficient Modulation for Band-Limited Channels," IEEE Journal on Selected Areas in Communications, Vol. SAC-2, No. 5, September 1984, pg. 637; and Lee-Fang Wei, "Trellis-Coded Modulation with Multidimensional Constellations," IEEE Transactions on Information Theory, Vol. IT-33, No. 4, July 1987, pp. 483-501. These methods are not generally applicable at all rates. They require a block encoder to partition "inner" and "outer" sets of points for selective transmission in one or more bauds of a larger block or frame. A unique encoder is required for each combination of bit rate and baud rate.
The commonly assigned, co-pending U.S. patent application of W. L. Betts et al entitled, "Modulus Converter for Fractional Rate Encoding," Ser. No. 588,658, filed Sep. 26, 1990, hereby incorporated by reference, discloses a method and apparatus which allows for, and is generally applicable to, any bit rate and baud rate combination. The baud rate is chosen to match the optimum bandwidth of the channel. A modulus, m, is computed from the smallest integer: EQU m.gtoreq.2.sup.(bit rate/baud rate).
Data is encoded for transmission using a signal constellation with m points. Each point in the constellation is identified by a unique integer index in the range 0 to m-1.
A frame length is selected by reducing the ratio of bit rate/baud rate to the lowest common denominator. The numerator identifies the number of bits in a frame and the denominator identifies the number of bauds in a frame, F. A modulus converter is used to convert the bits to F words of modulus m. Each word is used as the index to select the respective unique point in the constellation for transmission.